Garmin GPSMAP 496 Owner's Manual page 153

Whiteline
The GSD 20 or GSD 21 Sounder Module can help you determine
whether the bottom is hard or soft. When the sonar sound waves are
reflected back by the bottom, a hard bottom returns a stronger signal
than a soft bottom. A thin whiteline indicates a softer bottom, while
a thick whiteline indicates a harder bottom. Normally, a red line
is used to show the point where water meets the bottom. This line
follows the bottom contour, along with any significant objects lying
on the bottom. The unit uses the whiteline function to make the
bottom layer information easier to distinguish.
Thick
Whiteline: Hard
bottom
Whiteline On
GPSMAP 496 Owner's Manual
Thermoclines
A rough definition of a thermocline is a break in water where
the water temperature changes faster than the water above it.
Thermoclines are shown as the weakest colors.
One of the unique features offered by Garmin is See-Thru
technology, which allows the GPSMAP 496 to hear both weak and
strong signals simultaneously so as to identify fish returns under
the toughest conditions, such as suspended in thermoclines or even
hiding near structures. See-Thru technology allows the GSD 20 or
GSD 21 Sounder Module to see through thermoclines and helps
locate fish where they live.
Whiteline Off
S U S >
ETTING UP AND SING ONAR
U S
NDERSTANDING ONAR
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Thermocline
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